In digital radio communication distinction may be made between asynchronous and synchronous transfer. In the following, asynchronous and synchronous systems will be discussed, especially when the transfer takes place over short distances (up to approximately 200 m), for example within an office building in which devices for both transfer types are found in the office rooms.
Traditionally, digital radio communication has been provided in the air interface for synchronous radio terminals such as mobile telephones and digital cordless telephones with speech communication and ISDN services. Common to these types of systems is that the information is transmitted to a certain receiver within certain determined points in time in time division systems on a given radio frequency. Examples of time division systems are the so called TDMA (Time Division Multiple Access) systems in which information is transmitted in time slots at predetermined periodic distances in time for each channel on a certain carrier frequency.
In this case, two communication directions, often referred to as uplink and downlink can use carrier frequencies in different frequency bands, as is the case in cellular mobile telephony systems, in which the spacing between the uplink and downlink frequencies is usually 45 MHz. There is also a variant of TDMA called Multi Carrier Time Division Multiple Access/Time Division Duplex (MCTDMA/TDD) in the following called TDD. In this variant of TDMA the same carrier frequency is used in the uplink and the downlink, but one carrier frequency is used for several duplex channels, as is the case, for example in the DECT system mentioned above, which has 12 time slots in the uplink and 12 in the downlink, a total of 24 time slots on the same carrier frequency.
Radio systems for asynchronous transfer usually use so called packet transfer on a certain radio frequency but these packets are transmitted and received at random points in time from one transceiver to another transceiver.
Examples of asynchronous systems are so called "Local Area Data Networks" which use packet switching, directly on a radio link. A certain frequency spectrum (for example 1910-1920 MHz assigned by the federal commission FCC in the USA) has been reserved for asynchronous radio transfer. In this way the short access time for transmitting asynchronous packets may be made small. Several users in this case share the same frequency spectrum for asynchronous transfer, so that no frequency planning is necessary. On rare occasions, collisions, and thus blocking, occurs in the network between two users, and the packet concerned is retransmitted. Retransmission at certain points in time after transmission of asynchronous data packets is previously known, see, for example that disclosed in SE-A-9302067-5. Synchronous or periodic transfer means that information is transmitted and received at predetermined periodically recurrent points in time, as in the TDMA systems mentioned above. In case of radio transfer over short distances as in the example mentioned above from American standard, the frequency band between 1910 and 1920 MHz is assigned for the asynchronous transfer and the band between 1920 and 1930 MHz is assigned for synchronous transfer. These two bands are thus to be used separately.
When two transceivers are positioned very close to each other, for example on the same desk, and if they use adjacent frequency bands, they can disturb each other. This is valid, for example, for two 10 MHz frequency bands within 20 MHz being used by a synchronous and an asynchronous terminal, respectively, on the same table. It does not help to use carrier frequencies in different frequency bands as a transmission signal is so much stronger than a receive signal that the selectivity in the receive filters is not sufficient to separate the payload signal from the noise when the frequency spacing is less than 10 MHz.
Swedish patent application 9403036-8 describes rules for asynchronous and synchronous transmission of the same kind as is described here. This previous patent application, however, is not intended to solve a problem of interference between terminals located close to each other but describes a method for increasing the frequency accessibility for synchronous systems. Patent application 9403036-8 describes a method used in synchronous radio transfer, in which synchronous transfer has been adapted to the rules for asynchronous transmission so that synchronous transfer can take place in an asynchronous frequency band too, for example in the 1910-1920 MHz band in the USA. This known solution allows synchronized systems to used asynchronous frequency bands as well, but without providing protection from asynchronous terminals nearby. This prior art method is thus suited only when a synchronous system is used, for example, in a building, when it is known that there is no asynchronous system in the building.
Another prior art method is disclosed in "Cellular Digital Packet Data System Specification" Release 1.1, Jan. 19, 1995, which may be obtained from "CDPD Forum" (a well known association of companies in the USA). CDPD uses available frequencies in cellular TDMA mobile telephony systems for the transfer of packet data, that is, only entire available carrier frequencies are used for this purpose, and the spacing between the uplink and downlink frequency bands is 45 MHz.